Household fuel cells using hydrogen as fuels have become popular along with progress in their development. Furthermore, in recent years, fuel cell vehicles utilizing hydrogen as fuels have eventually been mass-produced and become commercially available in the same manner as household fuel cells. However, compared with household fuel cells for which existing city-gas and commercial-power-supply networks can be utilized, hydrogen infrastructures are indispensable for such fuel cell vehicles.
Therefore, it would be required that the number of hydrogen-filling stations serving as hydrogen infrastructures is expanded to further expand and popularize fuel cell vehicles in the days ahead. However, large-scale facilities and sites are required to build current hydrogen-filling stations, and thus, huge investments are required. This has been a major problem that remains to be solved in order to popularize fuel cell vehicles.
In order to cope with such a situation, development of an inexpensive compact, small-sized, household hydrogen-filling apparatus that serves as an alternative to a large-scale hydrogen-filling station has been expected. The most important issue for development of such a small-sized household hydrogen-filling apparatus is to develop a hydrogen compressor, and attention is currently focused on electrochemical hydrogen pumps that make it possible to electrochemically raise pressure of a hydrogen gas.
Compared with conventional mechanical hydrogen compressors, electrochemical hydrogen pumps have many merits. For example, they are compact and highly efficiently functional, they require little maintenance since they do not have any mechanically-operating parts, and cause almost no noise. Therefore, the development of commercially-viable electrochemical hydrogen pumps has eagerly been anticipated.
Currently, as one system that is expected to serve as a small-sized hydrogen-filling apparatus, a technique in which a hydrogen gas produced by use of a fuel-reforming device for household fuel cells is electrochemically compressed by use of an electrochemical hydrogen pump when the operation of the fuel cell is suspended can be mentioned. According to such an electrochemical hydrogen pump, besides the above-mentioned merits, the concentration of hydrogen that has been produced by use of the fuel-reforming device and that would be 75% at the highest can be reformed to a 100% hydrogen gas, and the hydrogen gas can be pressurized to an extreme pressure that makes it possible to fill the hydrogen gas into a fuel cell vehicle.
Moreover, the structure of the electrochemical hydrogen pump is almost the same as a structure of a power-generation stack in a household fuel ceil. A major difference between them is that, in contrast to eh anode side to which a low-pressure hydrogen gas is supplied, the pressure at the cathode-side needs to be equal to or higher than the extreme pressure that makes it possible to fill the hydrogen gas into the fuel cell vehicle, and therefore, a specific structure for supporting an electrolyte membrane present between both the electrode layers is required.
A structure of a power-generated stack 1 in a conventional fuel cell is shown in FIG. 1. In FIG. 1, an electrolyte membrane 2, on both surfaces of which an anode electrode layer 3 and a cathode electrode layer 4 are formed, is sandwiched between an anode diffusion layer 4 and a cathode diffusion layer 6, the resulting stack is further sandwiched between an anode separator 7 and a cathode separator 8, the resulting stack is further sandwiched between an anode insulation plate 11 and a cathode insulation plate 12, and then, the resulting stack is fixed by bolts 13 and nuts 14.
Furthermore, an anode seal 9 and a cathode seal 10 are attached onto the circumferences of the anode diffusion layer 5 and the cathode diffusion layer 6, respectively, to prevent the gas from leaking to the outside. When the power-generation stack 1 in the fuel cell is used as a hydrogen pump, an anode inlet 15 is used for supplying a low-pressure hydrogen gas to the anode side of the power-generation stack 1, an anode outlet 16 is used for recovering an excess low-pressure hydrogen gas from the anode side, and a cathode inlet 17 is used for retrieving a high-pressure hydrogen gas from the cathode side of the power-generation stack 1. However, a cathode inlet 18 is not used, and therefore, is sealed. By using the inlets and the outlets in such a manner, a low-pressure hydrogen gas is supplied to the power generation stack 1 through the anode inlet 15, and voltage is applied to the stack between the anode separator 7 and the cathode separator 8 by a voltage-applying unit 19 in a state where the low-pressure hydrogen gas is flowed through anode flow channels 7a. As a result, hydrogen is dissociated into protons and electrons at the anode electrode layer 3, as shown in Formula 1.Anode electrode layer:H2(low-pressure)→2H++2e−  (Formula 1)
The protons dissociated at the anode electrode layer 3 move to the electrolyte membrane 2 while entraining water molecules. Meanwhile, the electrons pass through the anode diffusion layer 5 and the anode separator 7, and move to the cathode separator 8, the cathode diffusion layer 6, and, eventually, the cathode electrode layer 4, through a voltage-applying unit 19. At the cathode electrode layer side, a reduction is taken place between the protons that have passed through the electrolyte membrane 2 and the electrons that has been moved from the cathode diffusion layer 6, and thus, hydrogen is produced. In that case, if the cathode inlet 17 is closed, the pressure of the hydrogen gas within the cathode flow channels 8b is increased, and thus, a high-pressure hydrogen gas is generated.Cathode electrode layer:2H+2+e−→H2(high-pressure)   (Formula 2)
In this case, a relationship among a pressure P1 of the hydrogen gas at the anode side, a pressure P2 of the hydrogen gas at the cathode side, and a voltage E is shown by Formula. 3 below.E=(RT/2F)ln(P2/P1)+ir  (Formula 3)
in Formula 3, R represents the gas constant (8.3145 J/K·mol), T refers to a temperature (K) of the cell, F refers to the Faraday constant (96485 C/mol), P2 refers to the pressure of the hydrogen gas at the cathode side, P1 refers to the pressure of the hydrogen gas at the anode side, i refers to a current density (A/cm2), and r refers to a cell resistance (Ω·cm2).
As apparent from Formula 3, it is understood that, when the voltage is increased, the pressure P2 of the hydrogen gas at the cathode side will increase.
However, it is required that an anode space 20 and a cathode-side space 21 are formed between the anode diffusion layer 5 and the anode seal 9, and between the cathode diffusion layer 6 and the cathode seal 10, respectively, in order to makes it possible to assemble these components without causing any problems.
That is, in order to embed a disk-shaped anode diffusion layer 5 having a diameter (φ) d shown in the perspective view of FIG. 2A into a hole (opening) of a ring-shaped anode seal 9 shown in the perspective view of FIG. 2B where the inner diameter (φ) of the hole (opening) is referred to by reference symbol D, it is required that the inner diameter D is larger than the diameter d. Hence, after embedding of the anode diffusion layer 5 into the anode seal 9, the anode-side space 20 having a width δ is generated as shown in FIG. 3. Additionally, the width δ of the anode-side space 20 is about half a difference between the diameter d and the inner diameter D. For example, when the diameter d of the anode diffusion layer 5 is 100 mm, the inner diameter D of the anode seal 9 is typical designed such that the width δ of the anode-side space 20 becomes about 0.1 mm.
This is because, when the difference between diameter d and the inner diameter D is designed to be excessively small, the above-mentioned length relationship between the diameter d and the inner diameter D is reversed due to manufacturing variations in dimensions of the anode diffusion layer 5 and the anode seal 9, and there would be cases where it becomes impossible to embed the anode diffusion layer 5 into the hole (opening) of the anode seal 9. Additionally, it would be considered that the produced anode diffusion layer 5 and the anode seal 9 may be subjected to a dimension-inspection process, and only acceptable materials may be used. However, if such a process is conducted, a problem in which yields of anode diffusion layers 5 and the anode seals 9 would be decreased and costs accordingly increase will arise. Therefore, it is required that an anode-side space 20 having a width of about 0.1 mm is formed between the anode diffusion layer 5 and the anode seal 9.
The same shall apply to the cathode diffusion layer 6 and the cathode seal 10.
When the power generation stack 1 of the fuel cell having the anode-side space 20 and the cathode-side space 21 as described above is used as a hydrogen pump to pressurize the hydrogen gas, the electrolyte membrane 2 is pressed toward the direction from the high-pressure side (cathode side) to the low-pressure side (anode side) due to the pressure of the hydrogen gas applied to the cathode-side space 21, as the pressure at the high-pressure side is increased. That is, the electrolyte membrane 2 is deformed so as to penetrate into the anode-side space 20 at the low-pressure side. If this deformation becomes excessive, cracks will be caused in the electrolyte membrane 2, and will eventually result in breakage thereof.
Hence, a pressure to which a hydrogen gas can be pressurized by using a power-generation stack 1 of a general fuel cell as a hydrogen pump is not very high, and it has been reported that such a power generation stack 1 is not completely effective at filling hydrogen into a fuel ceil vehicle. To solve this problem, adoption of a structure for supporting an electrolyte membrane has been proposed in cases where such a general power-generation stack is used as a hydrogen pump, so as not to cause breakage of the electrolyte membrane even if a substantial difference in pressures is present between the high-pressure side and the low-pressure side (WO2015/020065).
A schematic cross-sectional view of an electrochemical hydrogen pump 22 disclosed in WO2015/020065 is shown in FIG. 4. According to the disclosure, the electrochemical hydrogen pump 22 is configured so that a rigid body within the low-pressure area, i.e., the anode diffusion layer 5, is broader than a high-pressure-applying region, i.e., an area within the cathode seal 10. That is, positions of the anode-side space 20 and the cathode-side space 21 are different.
For the above reason, even when a high pressure is applied to the electrolyte membrane 2, the pressure can be received by the anode diffusion layer 5 having nigh rigidity and present at the low-pressure side. Accordingly, the electrolyte membrane 2 does not receive any bending force and shearing force that can cause breakage of the electrolyte membrane 2. Therefore, it is proposed that the electrolyte membrane 2 can safely be supported even if a substantial difference in the pressures is present.